html {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    height: 100vh;
}
body {
    display: flex;
    flex-direction: column;
    padding: 20px;
    margin: 0;
    width: 100%;
    height: calc(100vh - 52px);
    box-sizing: border-box;
}

footer {
    flex-grow: 1
}

#pack {
    height: 900px;
    background-color: rebeccapurple;
}